In Chapter 4, the Iran–Iraq War (1980–88) serves as a case-study to test (and ultimately refute) a purely enforcement-based formalization of belligerent reprisals. In the field of chemical warfare, reciprocity and enforcement are shown to converge (rather than exclude each other) in the operationalization of belligerent reprisals. Reciprocity is seen as inspiring both the purposes associated with the measure (restoring the balance of rights and obligations and countering unlawful military advantage) and the specific traits that it would take (in-kind breach). In the "war of the cities", belligerents resorted to reprisals with purposes that cannot be encapsulated in the enforcement paradigm. These included the function of ensuring equality of opportunities (as a form of negative reciprocity) and that of strengthening, enacting and agreeing on new standards of conduct when the specific content of applicable rules was not clear or settled (as an aspect of positive reciprocity). As a result, belligerent reprisals appear as a highly flexible tool by which parties to an armed conflict bargain, approve or refuse, and police the concrete legal framework governing wartime interactions.

Before the 20th century, most rules of international law were in the form of customary international law. Since then, the increased complexity of international relations and rapid international development have led to a substantial growth in the number and diversity of treaties. Article 38(1)(a) of the Statute of the International Court of Justice (‘ICJ Statute’) recognises treaties as a (material) source of international law by referring to ‘international conventions, whether general or particular, establishing rules expressly recognized by the contesting states’. Treaties now regulate trade, communications, environmental protection, military cooperation and defence, and human rights, to name but a few of the myriad topics. International environmental law, for example, is almost entirely governed by treaties, and international trade, investment and communications ‘are unimaginable without treaties’. The main rules in the law of treaties are contained in the 1969 Vienna Convention on the Law of Treaties (‘VCLT’), which governs treaty relations between states and is the focus of this chapter.

This chapter explains the role played by reservations, the application of the rules of the Vienna Convention on the Law of Treaties, and practice in dealing with reservations. It defines reservations and distinguishes them from interpretative declarations and political declarations. It examines whether reservations are permissible or impermissible, including the test whether a reservation is incompatible with the object and purpose of the treaty. It also discusses the rules governing acceptance and objection, the legal effect of reservations, the question whether a prohibited reservation can be severed, and special cases such as human rights treaties, reservations relating to internal law, vague or general reservations, reference to Islamic law, late reservations, and the role of treaty monitoring bodies. It analyses the procedure for making or objecting to reservations or withdrawing them, and the functions of the depositary in relation to reservations.

This chapter focuses on the rules set out in the 1969 Vienna Convention on the Law of Treaties (VCLT). The chapter begins with the concept of a treaty, before discussing treatymaking, with a particular focus on the conclusion of treaties, their entry into force, and reservations to treaties. The chapter then delves into how treaties operate -- namely, their scope of application and their interpretation. Finally, this chapter looks at the invalidity, suspension, and termination of treaties.

This chapter focuses on the rules set out in the 1969 Vienna Convention on the Law of Treaties (VCLT). The chapter begins with the concept of a treaty, before discussing treatymaking, with a particular focus on the conclusion of treaties, their entry into force, and reservations to treaties. The chapter then delves into how treaties operate -- namely, their scope of application and their interpretation. Finally, this chapter looks at the invalidity, suspension, and termination of treaties.